fix(openai_compat): improve prompt_cache_key host matching

[mahendrateja95]

Follow-up to #1353, addressing reviewer feedback from @yinwm:

1. Replaced strings.Contains(apiBase, "api.openai.com") with proper url.Parse() +
   url.Hostname() matching to prevent false positives from proxy URLs
   (e.g., https://proxy.com/api.openai.com/v1 would incorrectly match before this fix).

2. Added Azure OpenAI endpoint support (*.openai.azure.com) to the allowlist, since
   Azure OpenAI also supports prompt_cache_key.

3. Refactored test helper to reduce duplication in cache key test cases.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

AI provided the initial code draft. I reviewed, modified, and validated the changes.

🔗 Related Issue

Follow-up to #1353. Fixes #1333.

📚 Technical Context

• Reference URL:
  https://platform.openai.com/docs/guides/prompt-caching
• Reasoning:
  • strings.Contains() is unsafe — proxy URLs like https://proxy.com/api.openai.com/v1
    would false-positive on the substring match
  • url.Parse() + url.Hostname() extracts the actual host for reliable matching
  • Azure OpenAI (*.openai.azure.com) supports prompt caching and was missing from the
    original allowlist
  • New supportsPromptCacheKey() helper keeps the logic clean and testable

🧪 Test Environment

• Hardware: PC
• OS: Windows 11
• Model/Provider: Tested with unit tests against Mistral, Gemini, DeepSeek, Groq,
  Minimax, Ollama, OpenAI, and Azure OpenAI API bases
• Channels: N/A (provider-level fix)

📸 Evidence

Details

Tests added/updated:

• TestProviderChat_PromptCacheKeySentToOpenAI — verifies OpenAI still gets the field
• TestProviderChat_PromptCacheKeyOmittedForNonOpenAI — 6 non-OpenAI providers excluded
• TestSupportsPromptCacheKey — edge cases: Azure variants, proxy URLs, malformed inputs, empty strings

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

[yinwm]

LGTM! The changes improve the robustness of prompt_cache_key host matching and add Azure OpenAI support.

Key improvements:

1. Uses url.Parse() + Hostname() instead of strings.Contains() to prevent false positives from proxy URLs
2. Adds Azure OpenAI (*.openai.azure.com) support
3. Good test coverage for edge cases (proxy URLs, empty strings, malformed input)

Thanks for the contribution!